In order to reduce the cost, power consumption, weight, and volume of digital cameras, it is necessary to make the CCD image sensor as small as possible while maintaining the required resolution. This is typically accomplished by decreasing the pixel size. However, for optics having the same focal number, the smaller pixel receives less total light and consequently the sensor is less sensitive. One method of increasing light sensitivity is to incorporate an Indium Tin Oxide (ITO) electrode which is more transparent than polysilicon, and to design the pixel with asymmetric gates so that as much as possible of its light sensitive region is covered with ITO.
However, in full frame CCD image sensors, ITO electrodes employed to replace the second level polysilicon electrode can result in significant amounts of impurities leaching out of the ITO electrode and into the Silicon below, significantly altering the surface channel potentials. Both p- and n-type impurities are found to diffuse through the gate dielectric and into the silicon surface. The surface impurities form interface states that are populated during normal device operation, causing a further shift in the channel potentials. While the leaching out of these impurities does not result in catastrophic conditions for the device, it does present a situation in which the operating point of the devices shift with time. Also, the large density of surface states, caused by the impurities at the Silicon/Silicon Dioxide interface, cause signal loss to occur due to a phenomenon called "charge pumping."
A PhD. Thesis by Christianus Hermanus Leopold Weijtens at the Technical University of Eindhoven, entitled, "Indium Tin Oxide for Solid-State Image Sensors," attempted to use a silicon nitride (Si.sub.3 N.sub.4) layer within image sensors to prevent the leaching of material from ITO electrodes into the silicon below. There was a shortcoming in these attempts because: (1) the ITO electrode would not properly adhere to the Si.sub.3 N.sub.4 layer; and (2) the lack of hydrogen passivation of the silicon surface at the interface with the silicon dioxide gate dielectric.
Another issue which can effect pixel sensitivity within image sensors is that of light falling in regions of the pixel that are not light sensitive, such as the drain required for the formulation of the antiblooming LOD structure. This loss of sensitivity cannot be recovered by the mere substitution of a polysilicon electrode with an ITO electrode.
Still other problems exist in image sensing devices employing color filter patterns. These color filters are arranged in patterns in which the color filters overlap. In patterns such as the "BAYER" color filter pattern, to name just one, the non-overlapping areas do not allow light transmission. Typically these areas between the color filters are over a sensitive area of the pixel, leading to a sensitivity loss.
As can be seen by the foregoing discussion, there remains a need in the art for pixel architectures that result in pixel designs that improve the sensitivity of image sensing devices.